An organic thin film type solar cell uses a photoelectric conversion layer combining a p-type organic semiconductor polymer and an n-type organic semiconductor exemplified by fullerene, and is configured to provide charge separation when an exciton produced by incident light arrives at a contact point of the p-type organic semiconductor polymer and the n-type organic semiconductor.
In such an organic thin film type solar cell, a bulk heterojunction (BHJ) type photoelectric conversion layer is frequently used. This is referred to as a bulk heterojunction type organic thin film solar cell.
Such a bulk heterojunction type photoelectric conversion layer is formed by applying a mixed solution, which consists of a p-type organic semiconductor, an n-type organic semiconductor and suitable solvent, and drying the mixed solution. Then, during the course of drying the mixed solution, the p-type organic semiconductor material and the n-type organic semiconductor material respectively spontaneously undergo aggregation and phase separation, and as a result, a p-n junction with a large specific surface area is formed.
Meanwhile, since organic thin film type solar cells can provide high photoelectric conversion efficiency in an indoor environment with a low intensity light, the organic thin film type solar cells can establish a separate realm from the mainstream Si solar cells, and are highly promising.
However, when compared with Si solar cells, the organic thin film type solar cells have a low fill factor (FF), and therefore, under the conditions of actual use, the supply voltage is lowered. That is, as for an organic thin film solar cell, in order to obtain a high output voltage under the conditions of actual use, it is required to achieve a high fill factor, that is, it is required to increase the fill factor.
Thus, in order to increase the fill factor, for example, there has been proposed a method of inserting a TiOx hole blocking layer between a photoelectric conversion layer including P3HT (poly[3-hexylthiophene]) as a p-type organic semiconductor material and including PCBM ([6,6]-phenyl-C61 butyric acid methyl ester) as an n-type organic semiconductor material, and a negative electrode (first method). Furthermore, for example, there has also been proposed a method of applying PCBM on the negative electrode side of a photoelectric conversion layer including P3HT as a p-type organic semiconductor material and includes PCBM as an n-type organic semiconductor material (second method). Furthermore, for example, there has also been proposed a method of using cesium carbonate having very strong polarity in an underlayer that forms a photoelectric conversion layer including P3HT as a p-type organic semiconductor material and including PCBM as an n-type organic semiconductor material, and preferentially depositing PCBM on the underlayer side (negative electrode side) by utilizing the high affinity between cesium carbonate and PCBM (third method).